Liquid crystal device and manufacturing method therefor with anisotropic conductive adhesive connecting glass substrate and glass auxiliary substrate

ABSTRACT

A liquid crystal device and a manufacturing method thereof are described. The device comprises a liquid crystal panel and an auxiliary panel formed with an IC circuit for supplying driving signals to the liquid crystal device. The auxiliary substrate is separately provided with the circuit and the function thereof is tested in advance of the assembling with the liquid crystal panel. By this procedure, the yield is substantially improved.

This is a Divisional application of Ser. No. 07/575,442, filed Aug. 30,1990 now U.S. Pat. No. 5,130,833.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to liquid crystal device and manufacturingmethod for the same.

2. Description of the Prior Art

Liquid crystal devices have been broadly used. Particularly, due totheir light weights and small sizes, many compact electric appliancessuch as compact electric calculators, watches, lap-top word processorsand pocket television systems are provided with liquid crystal displays.

The driving circuit of such a liquid crystal display has been formed ina separate printed board made of a glass epoxy substrate in which asemiconductor device such as a packaged IC is mounted. The electricconnection is made through FPC (Flexible Print Circuit) between theprinted board and inner electrodes formed in the insides surfaces of apair of substrates in which a liquid crystal is disposed.

Recently, TAB (Tape Automated Bonding) has been employed to makeelectric connection. In accordance with this method, semiconductor chipsprovided with solder bumps on their contact pads are mounted on andelectrically connected with Cu electrodes formed on a polyimide film byface-down bonding (ILB: Inner Lead Bonding), and then electrode stripsformed on the substrates of a liquid crystal display are coupledrespectively with counterpart electrode strips of the polyimide film(OLB: Outer Lead Bonding).

The IC chips can be mounted directly on the substrates of a liquidcrystal display. In this case, an electric circuit pattern for supplyingdriving signals to electrode strips of the substrates defining pixels ofthe liquid crystal display and control signals to the chips has to beformed in advance. The IC chips are connected to the pattern by means ofAu wiring or bumps attached to the contact pads of the chips.

There are some shortcomings, however, in the above conventionaltechnique. In the case using a glass epoxy board, the advantage of lightweights claimed by liquid crystal display is largely diminished becauseof the heavy board. Furthermore, the use of the intermediary FPC isundesirable. There are two sites of connection at both ends of the FPCand therefore the frequency of occurrence of connection error isdoubled. The difference between the FPC and the glass substrates of aliquid crystal display in thermal expansion coefficient becomes a causeof stress at the connection. This problem is also the case at theconnection made in accordance with the TAB method. In addition,polyimide tape is expensive. In the case of the direct mounting of ICchips, the yield of production is the product of the yield of liquidcrystal panel and the yield of the driving circuit and therefore thedecrease of the whole yield is substantial.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a light weightliquid crystal device.

It is another object of the present invention to provide a method ofmanufacturing liquid crystal devices at a high yield.

Additional objects, advantages and novel features of the presentinvention will be set forth in the description which follows, and inpart will become apparent to those skilled in the art upon examinationof the following or may be learned by practice of the present invention.The object and advantages of the invention may be realized and attainedby means of the instrumentalities and combinations particularly pointedout in the appended claims.

To achieve the foregoing and other object, and in accordance with thepresent invention, as embodied and broadly described herein, there isprovided a third substrate beside a pair of substrates sandwiching aliquid crystal layer therebeween. The third substrate is provided withIC chips for producing driving signals to be supplied to electrodearrangement formed on the inside surfaces of the pair of substrates. Inaccordance with the present invention, no epoxy print board is used andtherefore the peculiar advantage of liquid crystal devices, i.e. Thelight weight, is not diminished.

As a method for electrically connecting the third substrate and the pairof substrates, there are several examples such as a method making use ofa UV light curable anisotropic conductive adhesive disposed between thethird substrate and the respective substrates, a method making use ofFPCs or a method making use of an anisotropic conductive rubber.

Furthermore, in accordance with the present invention, the liquidcrystal panels and the circuit boards for producing driving signals aremanufactured and tested separately and therefore inoperative boards canbe eliminated in advance of assembling process of liquid crystaldisplays so that the yield is expected to substantially increase.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe invention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a perspective view showing a liquid crystal display inaccordance with an embodiment of the present invention.

FIGS. 2(A) and 2(B) are plan views showing substrates formed withelectrode strips for liquid crystal display.

FIG. 3 is a partial cross sectional view showing electrical connectionbetween substrates carrying electrode strips in accordance with thepresent invention.

FIG. 4 is a perspective view showing a liquid crystal display inaccordance with a modification of the embodiment of the presentinvention shown in FIG. 1.

FIG. 5 is a perspective view showing a liquid crystal display inaccordance with another modification of the embodiment of the presentinvention.

FIG. 6 is a perspective view showing a liquid crystal display inaccordance with a further modification of the embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1, 2 and 3, a liquid crystal display inaccordance with the present invention is illustrated. The liquid crystaldisplay comprises a pair of glass substrates 1 and 2 between which aliquid crystal layer is disposed, and a pair of auxiliary substrates 3carrying thereon a driving circuit for supplying driving signals toorthogonal electrode strips 9 arranged in the inside surfaces of thesubstrates 1 and 2 as shown in FIG. 1. The auxiliary substrates 3 andthe substrates 1 and 2 are connected respectively at their edges inorder to establish electrical coupling between corresponding electrodes.

Next, a method of manufacturing the liquid crystal display in accordancewith a first embodiment is explained. First and second sodalime glasssubstrates of 1.1 mm thickness are coated with transparent conductivefilms such as ITO (Indium Tin Oxide) films by DC magnetron sputtering toa thickness of 1200 angstroms. The conductive films are patterned bymeans of laser pulses emitted from an excimer laser into a number ofparallel electrode strips as shown in FIG. 2(A). The number of electrodestrips of the first substrate is 640 while that of the second is 400since the shape of the substrates is rectangular and the electrodestrips have to be orthogonally arranged when the substrates are joinedin order to define pixels therebetween at the intersections of theorthogonal strips. The output energy of the excimer laser beam is 250mJ. The pulse width is 20 nanoseconds. The cross section of the beam is15 μm×400 mm. The repetition frequency is 10 Hz.

The laser scribing is carried out in the same manner again for thepurpose of insuring the insulation between adjacent terminals of theelectrodes. Namely, after the electrodes are formed, the laser scribingis carried out in order to reduce the width of the ends of theelectrodes as shown in FIG. 2(B) with a mask covering the centerportions thereof. The cross section of the excimer laser beam in thiscase is 30 μm×400 mm. The other lasing conditions are same as those ofthe above. The surfaces of the substrates provided with the electrodesare then coated with polyamic acid by the offset printing. The polyamicacid is then cured in a clean oven at 350° C. for three hours to formpolyimide thin films. The polyimide film on the first substrate is givenrubbing treatment by means of a cotton cloth and SiO₂ fine particles of8 μm diameter are dusted thereon as spacers. The second substrate isgiven an adhesive at its perimeter and joined to the first substrate.The inside space between the first and second substrates are filled witha liquid crystal material by a known vacuum injection. The opening,through which the liquid crystal material is injected, is then sealedoff by an adhesive cured by ultraviolet rays. One side of each of thefirst and second substrates extends together with the electrode stripsand is exposed beyond the other substrate in order to provide contactsfor electrically connection with the driving circuits formed on theauxiliary substrates 3 as shown in FIG. 1.

The auxiliary substrates 3 are provided by depositing ITO films on 1.1mm thick sodalime glass substrates of 50 mm width and 270 mm length. TheITO film is patterned by a known photolithography. The ITO pattern isthen coated with Ni and thereafter with Au respectively by plating. Thepattern includes contacts with which IC chips for producing drivingsignals are electrically connected. The connection of the chips and thecontacts are made by applying Au bumps to contact pads of the chips,dripping a curable adhesive onto the rare surfaces of the chips and,after aligning the pads and the contacts of the substrate, exposing theadhesive to ultraviolet light under pressure of 95 g per bump at 150° C.for three minutes in order to harden the adhesive and fix the chips tothe substrate. Then, the driving circuits as manufactured as above onthe substrate are tested at the outer leads with which the electrodes ofa liquid crystal display should be connected for supplying drivingsignals in order to eliminate substrates carrying inoperative circuits.

The electric connection between the first and second substrates 1 and 2and the counterpart auxiliary substrates 3 respectively are done asfollows. The extended inside surfaces of the first substrates 1 on whichterminals of the respective electrodes are exposed are coated, by meansof a dispenser, with an anisotropic conductive film. The adhesive filmis made from a UV light curable adhesive 8 in which a number ofresilient fine conductive particles 6 and hard particles 7 whosediameter is slightly smaller than that of the resilient particles aredispersed. The resilient particles are made from 7.5 μm thickpolystylene spheres plated with a 1000 angstroms thick Au film. The hardparticles are made from 5 μm thick SiO₂ spheres. The weight proportionamong the adhesive, the resilient particles and the hard particles is107:14:1. Then, the first substrate 1 and the auxiliary substrates 3 arejoined with the adhesive therebetween in order that the the terminals ofthe first substrate 1 and the corresponding contacts of the auxiliarysubstrate 3 are aligned to each other, and exposed to UV light for 3minutes under pressure of about 2.4 kg/cm². The hard particles areincluded in order to prevent the conductive resilient particles frombeing destroyed under excess pressure. The second substrate 2 iselectrically connected with the other auxiliary substrate 3 in the samemanner. When displays as produced as above were subjected to a thermalshock test at -30° C. and at 70° C. respectively for an hour, all thesample displays were maintained operatable.

Next, a method of manufacturing the liquid crystal display in accordancewith a second embodiment is explained in conjunction with FIGS. 1, 2(A)and 3. First and second sodalime glass substrates of 1.1 mm thicknessare coated with transparent conductive films such as ITO (Indium TinOxide) films by DC magnetron sputtering to a thickness of 1200angstroms. The conductive films are patterned by means of laser pulsesemitted from an excimer laser into a number of parallel electrode stripsas shown in FIG. 2(A). The number of electrode strips of the firstsubstrate is 640 while that of the second is 400 since the shape of thesubstrates is rectangular and the electrode strips have to beorthogonally arranged when the substrates are joined in order to definepixels therebetween at the intersections of the orthogonal strips. Theoutput energy of the excimer laser beam is 250 mJ. The pulse width is 20nanoseconds. The cross section of the beam is 15 μm×400 mm. Therepetition frequency is 10 Hz.

In this case, the treatment of terminals of the electrode strips 9 isnot carried out. A liquid crystal panel is assembled in the same manneras the above embodiment except that the first and second substrates areprovided with the electrode strips as illustrated in FIG. 2(A) ratherthan FIG. 2(B). A pair of auxiliary substrates are provided also in thesame manner as the above embodiment.

The electric connection between the first and second substrates 1 and 2and the counterpart auxiliary substrates 3 respectively are done asfollows. The extended inside surfaces of the first substrates 1 on whichterminals of the respective electrodes are exposed are coated, by meansof a dispenser, with an anisotropic conductive film. The adhesive filmis made from a UV light curable adhesive 8 in which a number ofresilient fine conductive particles 6 and hard particles 7 whosediameter is slightly smaller than that of the resilient particles aredispersed. The resilient particles are made from 2.5 μm thickpolystylene spheres plated with a 1000 angstroms thick Au film. The hardparticles are made from 2 μm thick SiO₂ spheres. The weight proportionamong the adhesive, the resilient particles and the hard particles is98:13:3. Then, the first substrate 1 and the auxiliary substrates 3 arejoined with the adhesive therebetween in order that the the terminals ofthe first substrate 1 and the corresponding contacts of the auxiliarysubstrate 3 are aligned to each other, and exposed to UV light for 3minutes under pressure of about 2.4 kg/cm². The hard particles areincluded in order to prevent the conductive resilient particles frombeing destroyed under excess pressure. The second substrate 2 iselectrically connected with the other auxiliary substrate 3 in the samemanner. When displays as produced as above were subjected to a thermalshock test at -30° C. and at 70° C. respectively for an hour, all thesample displays were maintained operatable.

Next, a method of manufacturing the liquid crystal display in accordancewith a third embodiment is explained in conjunction with FIGS. 1 and2(A). First and second sodalime glass substrates of 1.1 mm thickness arecoated with transparent conductive films such as ITO (Indium Tin Oxide)films by DC magnetron sputtering to a thickness of 1200 angstroms. Theconductive films are patterned by a known photolithography into a numberof parallel electrode strips as shown in FIG. 2(A). The number ofelectrode strips of the first substrate is 640 while that of the secondis 400 since the shape of the substrates is rectangular and theelectrode strips have to be orthogonally arranged when the substratesare joined in order to define pixels therebetween at the intersectionsof the orthogonal strips.

The surfaces of the substrates provided with the electrodes are thencoated with polyamic acid by the offset printing. The polyamic acid isthen cured in a clean oven at 350° C. for three hours to form polyimidethin films. The polyimide film on the first substrate is given rubbingtreatment by means of a cotton cloth and SiO₂ fine particles of 8 μmdiameter are dusted thereon as spacers. The second substrate is given anadhesive at its perimeter and joined to the first substrate. The insidespace between the first and second substrates are filled with a liquidcrystal material by a known vacuum injection. The opening, through whichthe liquid crystal material is injected, is then sealed off by anadhesive cured by ultraviolet rays. One side of each of the first andsecond substrates extends together with the electrode strips and isexposed beyond the other substrate in order to provide contacts forelectrically connection with the driving circuits formed on theauxiliary substrates 3 as shown in FIG. 1. Then, a pair of auxiliarysubstrates are provided, tested and coupled with the liquid crystalpanel in the same manner as the above embodiment.

FIG. 4 illustrates a modification of the embodiment illustrated inFIG. 1. The liquid crystal panel is same as that of the firstembodiment. One end of each auxiliary substrate 3 is formed to extend inorder to join with the other substrate 3. The extended ends of thesubstrates are fixed to each other by means of a UV light curableadhesive as shown in FIG. 4. Since the distance between the joined endsof the substrates 3 equals the distance between the substrate 1 and 2which are adhered in the same manner, this procedure can be naturallydone without any dimmension disagreement.

FIG. 5 illustrates another modification of the embodiment illustrated inFIG. 1. The liquid crystal panel is same as that of the firstembodiment. A reinforcement plate 18 is attached to the auxiliarysubstrate 3 and the substrate 2 astride the gap therebetween. Since thesubstrates 3 and 2 have their upper surfaces flush with each other, thisprocedure can be naturally done without any dimmension disagreement.

FIG. 6 illustrates a further modification of the embodiment illustratedin FIG. 1. The liquid crystal panel and the auxiliary substrates aresame as that of the first embodiment. The connection therebetween isdone through FPCs. Namely, the substrate 1 or 2 is coupled with an FPC11 by means of an anisotropic conductive film 14. At the other end ofthe FPC 11, the auxiliary substrate 3 is coupled with the FPC also bymeans of the anisotropic conductive film 14. The conductive film 14comprises a thermally curable adhesive which adheres the FPC 11 to thesubstrate by thermal treatment at 200° C. for 8 seconds under pressureof 30 kg/cm². By this procedure, the electrodes of the substrates 1 and2 are coupled with the corresponding electrodes 13 of the auxiliarysubstrates 3 respectively through Cu conductive strips 15 formed on theFPC 11.

The foregoing description of preferred embodiments has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form described, andobviously many modifications and variations are possible in light of theabove teaching. The embodiment was chosen in order to explain mostclearly the principles of the invention and its practical applicationthereby to enable others in the art to utilize most effectively theinvention in various embodiments and with various modifications as aresuited to the particular use contemplated. Although in the abovedescription a pair of substrates sandwiching a liquid crystal layer areprovided with electrode strips and connected with a pair of auxiliarysubstrates on which IC chips are mounted respectively to supply drivingsignals, there are devices which require driving signals only at one ofthe pair of the substrates. In this case, the auxiliary substrates areneeded only one for one liquid crystal panel.

What is claimed is:
 1. A liquid crystal device comprising:first and second glass substrates; electrode arrangements respectively formed on inside surfaces of said substrates; a liquid crystal layer disposed between said first and second substrates; and circuits respectively formed on at least one glass auxiliary substrate and respectively connected with at least one of said electrode arrangements for supplying driving signals to said one electrode arrangement wherein said electrical connection is made through an anisotropic conductive adhesive fixing said auxiliary substrate in a predetermined position with respect to said electrode arrangement and simultaneously providing an electrical connection between both (a) said circuit formed on the auxiliary substrate and (b) the one electrode arrangement formed on the inside of one of the substrates, wherein at least one of said first and second substrates extends beyond the edge of the other in order to provide terminals of said one electrode arrangement with which said circuit makes electric contact.
 2. The liquid crystal device of claim 1 wherein each of said first and second substrates is extending beyond the edge of the other in a different direction in order to provide terminals of said electrode arrangement with which said circuit makes electric contact.
 3. The liquid crystal device of claim 1 wherein the number of said auxiliary substrates is two, one being connected to the first substrate and the other to the second substrate.
 4. The liquid crystal device of claim 3 wherein said auxiliary substrates have extended portions which are joined to reinforce the mechanical strength of said device.
 5. A liquid crystal device comprising:first and second glass substrates; an electrode arrangement formed on the inside surfaces of said substrates; a liquid crystal layer disposed between said first and second substrates; and a circuit formed on at least one glass auxiliary substrate and connected with said electrode arrangement for supplying driving signals to said electrode arrangement wherein said electrical connection is made through an anisotropic conductive adhesive which contacts both (a) said circuit formed on the auxiliary substrate and (b) the electrode arrangement formed on the inside of one of the substrates.
 6. A liquid crystal device comprising:first and second glass substrates; electrode arrangements respectively formed on inside surfaces of said substrates; a liquid crystal layer disposed between said first and second substrates; and at least one glass auxiliary substrate on which semiconductor chips and electrical wiring for supplying driving signals to said liquid crystal layer are formed, wherein said first substrate extends beyond the edge of said second substrate in order to provide terminals of said electrode arrangement; and wherein said auxiliary substrate is directly attached to the extended portion of the first substrate through an anisotropic conductive adhesive so that said electrical wiring formed on said auxiliary substrate is electrically connected to said terminals.
 7. The device of claim 6 wherein a reinforcement plate is attached over external surfaces of said auxiliary substrate and said second substrate.
 8. The device of claim 6 wherein said auxiliary substrate has substantially the same thickness as said second substrate. 